Gives the ifIndex of the SONET port that this path is actually running over right now, which can differ from the port it was originally configured on if an APS switch has moved it. Admins use it to trace where a given path's traffic is physically flowing at any moment, especially useful after a protection switch has relocated the path onto a different physical port. If a path was provisioned on port ifIndex 5 but eqSonetPathActualPort now reports 6, an admin knows an APS switch has moved that path's traffic onto port 6's protection line.